Thanks guys, I have a cheapy Tevion head unit which has bluetooth, usb, sd slot so I'm planning on keeping that, I believe it is 45Wx4. I have a sub and amp in the boot which is loud enough to vibrate the car but I just find the fronts don't match up to it. I just managed to acquire a pair of small Blaupunkt speakers for free but I'm not sure what exact size or power they are. I will try the above advice and hopefully they will fit. I had some JVC ones before but they wouldn't fit.
Ideally I don't want to spend anything at the minute as I've just bought a bunch of parts and a younger donor car but if these speakers don't fit I guess I'll have to make a new plan!